Local air-fuel ratio and gas velocity measurements in flames using LIBS

Abstract

Turbulent combustion is an important but complex technical process that takes place in many industrial devices, such as internal combustion engines and gas turbines. Laser-based techniques are widely applied to the combustion research, since they are able to provide important information without severe interferences with the process and with high temporal and spatial resolution. Among these techniques, Laser-induced Breakdown Spectroscopy (LIBS) is an experimental technique that can deliver a manifold of information through the laser-induced plasma. In the current work, laser-induced plasma is used to develop novel diagnostic methods for the measurement of the gas composition and velocity in flames. This research firstly evaluated and improved the instantaneous LIBS measurement of local equivalence ratio in methane-air opposed-jet flames. It was found that the increased measurement uncertainty in reacting flows, relative to non-reacting flows, could be counteracted by increasing the laser pulse energy and using a proposed filter method, which resulted in a 10% uncertainty for single-shot measurements in flames. LIBS measurements in binary hydrocarbon mixtures, including CH4, C3H8, CO2 and air, were also investigated. The effects of blends on LIBS spectra were analysed. While limitations exist, it was shown that LIBS results can be used to infer the binary fuel blends and the Wobbe index of the mixture. Further, a novel seeding-free velocimetry technique, Laser-Induced Plasma Image Velocimetry (LIPIV) was proposed and developed, which is based on recording the displacement of plasma as it gets convected by the gas flow field. It was found that the uncertainty of the novel technique is about 1 m/s along the axis perpendicular to the direction of the laser beam and 3.6 m/s along the laser beam axis and its spatial resolution is about 5 mm. This technique was then successfully applied to measure the gas flow in a non-reacting turbulent jet. Finally, local equivalence ratio and velocity measurements were performed in a swirl-stabilised non-premixed burner at an air-fuel ratio of 0.72 (overall equivalence ratio) and Reynolds number of 29,000. It was shown that, although the quenching effects and plasma deformation effects increased the measurement uncertainty, the velocity results still verified the feasibility of applying the LIPIV in reacting flows after applying the proposed methods. In conclusion, combined measurements of composition and velocity using the laser-induced plasma are demonstrated in realistic combustion geometries.